Sample discriminating method

ABSTRACT

A glucose sensor system comprising the steps of using as a sample discriminating parameter a ratio (I/ΔI) of a measured current value I to the time-differential value of the current value ΔI, defining a discrimination function that discriminates whether a sample is blood or control fluid and uses the discriminating parameter as an independent variable, quantitating as a discriminating index a numeric value obtained by substituting a discriminating parameter value into this discrimination function, and automatically discriminating, based on this index, whether the sample is blood or a control fluid, whereby a kind of the sample can be automatically quantitated by measuring electric current when a sensor system is used for quantitating the concentration of an analysis object in the sample.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/JP00/08393 which has an Internationalfiling date of Nov. 29, 2000, which designated the United States ofAmerica and was not published in English.

TECHNICAL FIELD

The present invention relates to a method of discriminating a sample fora sensor system which measures the concentration of a target substance(substance of interest) contained in the sample, in particular for abiosensor system which quantitates the concentration of glucose,cholesterol or the like contained in a body fluid such as blood bymeasuring electric current. More particularly, it relates to a samplediscriminating method of automatically discriminating whether the sampleintroduced to a sensor system is a body fluid or a standard fluid, thesensor system being designed to periodically examine whether theaccuracy of the sensor system is maintained or not, using the standardfluid, whose concentration has been known, instead of the body fluid.

BACKGROUND ART

There has been well known a sensor system which quantitates theconcentration of a target substance contained in a sample by measuringelectric current. In recent years, the sensor system described above hasbeen widely applied to, for example, a biosensor system such as a smalland easy type of blood sugar measuring system for quantitating the valueof blood sugar contained in blood, or the like. Recently, the bloodsugar measuring system, which is used for a blood sugar diagnosis ordaily management of a diabetes patient, is commercially prevailed whilebeing provided with many various functions. Recently, for example, theblood sugar measuring system is particularly of great importance in thefield of a data management such as a management or processing ofmeasured data.

In general, measurement accuracy of the biosensor system including asensor and a measuring device is periodically managed using, forexample, an exclusive standard fluid (referred to “control fluid”hereinafter), in order to maintain or manage the measuring accuracy. Asthe control fluid, there may been used a solution in which a knownamount of glucose is dissolved in pure water, the solution being coloredwith a pigment in accordance with its use, or being provided with ahydrophilic polymer so as to adjust its viscosity.

In the conventional biosensor system in which its measurement accuracyis managed using the control fluid, it is required that the measureddata of the control fluid is not confusedly processed as the measureddata of the body fluid or the like used as an ordinary sample.Accordingly, before the control fluid is introduced into the biosensorsystem, the measuring mode is changed to that for the control fluid by apredetermined manual operation of the measuring device so as todistinguish its measuring data from the measuring data of the body fluidor the like.

However, in the conventional biosensor system described above, when thecontrol fluid is introduced, it is required to change the measuring modeby the manual operation for changing the mode, for example, buttonoperation or the like. In consequence, there is such a problem that themeasured data for the control fluid may be managed while erroneouslybeing recognized as the measured data for the body fluid or the like byerroneously performing or forgetting the operation. Meanwhile, there maybe also such a problem that the operation for manually changing the modeis troublesome. In particular, for a diabetes patient or the like havingtrouble in the eyes or fingertips, it may be difficult to change themode by the manual operation. Therefore, there is requested a biosensorsystem, which can automatically discriminate whether a sample introducedinto the biosensor system is a body fluid or a control fluid.

DISCLOSURE OF INVENTION

The present invention, which has been developed to solve theconventional problems described above, has an subject to provide a meanswhich can automatically discriminate the kind of a sample for a sensorsystem that quantitates the concentration of a target substancecontained in the sample by measuring electric current, for example, abiosensor system such as a blood sugar measuring system.

A sample discriminating method according to the present invention whichhas been developed to achieve the above-mentioned object, is a method ofdiscriminating a sample for a sensor system which quantitates theconcentration of a target substance (substance of interest) contained inthe sample by measuring electric current, the method comprising thesteps of, (i) using a ratio of a measured current value to atime-differential or time-difference value of the current value as adiscriminating parameter, (ii) defining a discrimination function fordiscriminating kinds of a plurality of objective samples, thediscrimination function using the discriminating parameter as anindependent variable, (iii) using a numeric value obtained bysubstituting the value of the discriminating parameter into thediscrimination function as a discriminating index, and (iv)automatically discriminating the kind of any sample based on thediscriminating index.

As the discriminating function, for example, there may be given adiscriminant function, a Mahalanobis distance or the like.

According to the sample discriminating method of the present invention,because the kind of the sample can be automatically discriminated, thekind of the sample may not be erroneously recognized due to erroneouslyperforming or forgetting the operation. Further, because it is notnecessary to change the mode by a manual operation, even a person havingtrouble in the eyes or fingertips can easily use the sensor system.

The discrimination function may be defined by means of an equation usingonly one discriminating parameter or independent variable. However, inorder to raise the accuracy of the discrimination, it is more preferablethat the discrimination function is defined by means of an expressionusing a plurality of, for example two, discriminating parameters orindependent variables.

The discrimination function may be defined by means of a linearexpression for the discriminating parameter. Meanwhile, thediscrimination function may be defined by means of a expression of highdegree, for example an expression of nth degree (n=2, 3, 4, . . . ), forthe discriminating parameter.

The sample discriminating method according to the present invention isparticularly effective for such a case that the kinds of the samples tobe discriminated are a body fluid such as blood and a control fluid. Inthis case, it is preferable that the sensor system is automaticallyjudged whether it is right or not, namely the system is checked, basedon a quantitated value of the concentration of the target substance inthe control fluid, and then a resultant judgement is indicated.

Meanwhile, in the sample discriminating method according to the presentinvention, it is preferable that when the value of the discriminatingindex is within such a predetermined range that it is difficult todiscriminate the kind of the sample, namely it exists within a regionnear a boundary, the kind of the sample is not automaticallydiscriminated while it is indicated that the discrimination has not beenperformed. If so, the accuracy or preciseness of the automaticdiscrimination of the kind of the sample may be highly improved.

Although it may be a rare case that the kind of the sample is notautomatically discriminated so that it is indicated that thediscrimination has not been performed, the kind of the sample may bedesignated by a manual operation in the above-mentioned case.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is an exploded perspective view of a sensor in a glucose sensorsystem using a sample discriminating method according to the presentinvention.

FIG. 1B is a perspective view of a biosensor system including the sensorshown in FIG. 1A and a measuring device.

FIG. 2A is a graph showing the relation between the applied voltage orits applying pattern and the time, when the value of the electriccurrent is measured in the glucose sensor system according to thepresent invention.

FIG. 2B is a graph showing a characteristic of the change of theelectric current with lapse of time after the application of the voltagehas been started again, in the case that the voltage is applied as shownin FIG. 2A.

FIG. 3 is a view obtained by plotting each of the sample data accordingto an embodiment of the present invention, based on the discriminatingparameter.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of the present invention will be concretelydescribed in detail with reference to the accompanied drawings.

FIGS. 1A and 1B show a glucose sensor system for quantitating theconcentration of glucose contained in a blood sample, namely the bloodsugar value, as an example of biosensor systems, the system beingsubstantially composed of a sensor 11 and a measuring device 12.

As shown in FIG. 1A, in the sensor 11 of the glucose sensor system, onan insulating support 6 made of PET (polyethylene terephthalate), thereare formed a first silver lead 4, a second silver lead 5, and anelectrode section including a working electrode 1 and a counterelectrode 2, each of which is made of carbon by means of screenprinting. Thus, the first silver lead 4 is electrically connected to theworking electrode 1 while the second silver lead 5 is electricallyconnected to the counter electrode 2.

Hereupon, the electric conductor composed of the first silver lead 4 andthe working electrode 1 is not electrically connected, in a directstate, to the electric conductor composed of the second silver lead 5and the counter electrode 2. However, they are electrically connected toeach other through a reactive layer 10 described below.

Moreover, an insulating layer 3 is formed at an upper side of theelectrode section, namely the working electrode 1 and the counterelectrode 2. Hereupon, the insulating layer 3 also covers a part of thefirst silver lead 4. Meanwhile, within a region in which the workingelectrode 1 is formed if seen in the plane view, a cutoff section 3 a isprovided in the insulating layer 3. Therefore, a portion of the workingelectrode 1, which corresponds to the cutout section 3 a, is exposedoutward. The insulating layer 3 with the cutout section 3 a is providedin order to make the exposed area of the working electrode 1 and thecounter electrode 2 become constant.

The reactive layer 10 is disposed on the insulating layer 3 and theelectrode section formed as described above. The reactive layer 10includes a layer of CMC (carboxylmethyl cellulose) which is one ofhydrophilic polymers, GOD (glucose oxidase) which acts as an enzyme, andpotassium ferricyanide which acts as a mediator. Further, on those,there is disposed an insert 9 composed of a cover 7 and a spacer 8.Then, when a sample fluid is made contact to the insert 9, the sample ofa constant amount, for example about 3 μL is introduced into thereactive layer 10 and the electrode section by means of a capillaryphenomenon.

Thus, as shown in FIG. 1B, after the sensor 11 has been mounted on themeasuring device 12, an electric source of the measuring device isturned on so that the device becomes such a state that it can receive asample, namely a blood sample or a control fluid. When the sample isapplied to the sensor 11, the voltage applied to the sensor 11 is shutonce, and then the reaction is incubated for a predetermined time. Afterthat, the voltage is applied again. Hereupon, the voltage is appliedbetween the first silver lead 4 and the second silver lead 5, furtherbetween working electrode 1 and the counter electrode 2.

In consequence, electric current, which corresponds to the concentrationof glucose contained in the sample, flows between the working electrode1 and the counter electrode 2 through the reactive layer 10, while theelectric current value is measured. Then, the concentration of glucosecontained in the sample is quantitated on the basis of the electriccurrent value.

In the glucose sensor system, the concentration of glucose contained ineach of various kinds of blood samples is measured or quantitated. Onthe other hand, in order to maintain the measuring accuracy, themeasuring accuracy is periodically managed using a control fluid, forexample a glucose standard solution. That is, a control fluid whoseglucose concentration has been known is used as a sample, while theglucose concentration is measured or quantitated. So, on the basis of anerror or the like of the quantitated value, the preciseness of theresultant value measured by the glucose sensor system is examined.Hereupon, as the control fluid, there may been used a solution in whichknown amount of glucose is dissolved in pure water, the solution beingcolored with a pigment in accordance with its use, or being providedwith a hydrophilic polymer so as to adjust its viscosity.

Then, in the glucose sensor system, a blood sample or a control fluid isreceived in the sensor 11 as a sample. In the glucose sensor system, themeasuring device 12 automatically discriminates or judges whether thesample actually received in the sensor 11 is a blood sample or a controlfluid. Accordingly, the kind of the sample may not erroneouslyrecognized due to erroneously performing or forgetting the operation.Further, because it is not necessary to manually change the operationmode, even a person having trouble in the eyes or fingertips can easilyuse the glucose sensor system.

Hereinafter, the sample discriminating method for the measuring device12 will be concretely described. The outline of the samplediscriminating method in the measuring device 12 is as follows.

(1) There is prepared a ratio of electric current value to atime-differential or time-difference value of the electric current valueas a sample discriminating parameter, the electric current value havingbeen measured for a blood sample or a control fluid.

(2) There is defined a discrimination function for discriminatingwhether the sample to be measured is a blood sample or a control fluid,the discrimination function using the discriminating parameter as anindependent variable.

(3) There is provided a numeric value obtained by substituting the valueof the discriminating parameter into the discrimination function as adiscriminating index.

(4) It is automatically discriminated whether the sample is a bloodsample or a control fluid on the basis of the discriminating index.

In the present embodiment, a general expression indicated by Expression1 described below is used as the discrimination function.

Z=a ₁ ×α+a ₂ ×β+a ₀  Expression 1

Z: discriminating index

α: first discriminating parameter (independent variable)

β: second discriminating parameter (independent variable)

a₁, a₂, a₀: constant

Thus, it is judged whether the sample is a blood sample or a controlfluid, for example, based on the conditions described below, using thediscriminating index Z calculated in accordance with the discriminationfunction indicated by Expression 1. Hereupon, L and H described belowmean the lower limit and upper limit of the area or range in which theabove-mentioned judgement is particularly difficult, respectively.

(1) In the case of Z<L, it is judged that the sample is a control fluid.

(2) In the case of L≦Z≦H, the judgement is not performed while it isdecided that the sample is un-decidable.

(3). In the case of Z>H, it is judged that the sample is a blood sample.

The discriminating parameter, which is the independent variable in thediscrimination function indicated by Expression 1, namely the ratio ofthe measured electric current value I to its time-differential ΔI(referred to “I/ΔI” hereinafter), is defined as follows. Hereupon, “I”means the electric current value at the time point that t seconds havepassed from the time point when the application of voltage has beenstarted again (starting point of the second voltage application).However, in the case that it is particularly necessary to clearlyindicate “t seconds have passed”, it will be described as I_(t).

“ΔI” means the absolute value of the difference |I_(t)−I_(t+Δt)| betweenI_(t) and the electric current value (I_(t+Δt)) at the time point thatrelatively short time Δt seconds have passed from the time point t atwhich I_(t) has been measured, namely at the time point that (t+Δt)seconds have passed from the starting point of the second voltageapplication. Alternatively, “ΔI” may mean the absolute value of thedifference |I_(t)−I_(t−Δt)| between I_(t) and the electric current value(I_(t−Δt)) at the time point that Δt seconds precede to the time point tat which I_(t) has been measured, namely at the time point that (t−Δt)seconds have passed from the starting point of the second voltageapplication. Each ΔI can be similarly used as a parameter whichindicates the degree or magnitude of the inclination of the wave shapeof the electric current near the time point that t seconds have passedfrom the starting point of the second voltage application.

In the example described above, the ratio between I and ΔI is a value ofI/ΔI (I÷ΔI) which is obtained when I is divided by ΔI. However, as theratio between I and ΔI, there may be used a value of ΔI/I (ΔI÷I) whichis obtained when ΔI is divided by I. In either case, the ratio can besimilarly used as the discriminating parameter, even though thediscrimination functions of those, which have been previously defined,are different from each other. Hereupon, the numerical value of thediscriminating parameter itself reflects the property of the kind of thesample fluid. Therefore, the sample can be discriminated by merelycomparing the numerical value simply to a standard value without usingthe discrimination function. However, in this case, there may remainsuch a problem that the accuracy of the discrimination of the sample islowered a little.

I/ΔI at the time point that t seconds have passed from the startingpoint of the second voltage application (referred to “I/ΔI(t)”hereinafter), can be similarly used as the discriminating parameter,regardless of the value of t, namely at every time point after themeasurement of electric current value has been started. However, it ispreferable to use I/ΔI(t) at a current decay point in which the propertyof the kind of the sample fluid is well reflected relatively. The timepoint or frequency for calculating t and Δt may be changed in accordancewith the composition of the control fluid to be discriminated.

FIG. 2A is a graph showing the relation between the applied voltage andthe time when the value of the electric current flowing actually throughthe sample in the reactive layer 10 is measured, that is, showing aconcrete process for applying the voltage between the both silver leads4,5 namely between the both electrodes 1,2. That is, as shown in FIG.2A, the voltage of 500 mV is applied before the sample is supplied.Thus, after the sample has been supplied at the time point of t=0, thesystem is set to an open circuit state for 25 seconds so that thevoltage application is shut. Then, the voltage of 500 mV is appliedagain for 5 seconds.

FIG. 2B is a graph showing an example of the resultant data of theelectric current (wave shape of the current) measured after the voltagehas been applied again, in the case that the voltage is applied in thepattern shown in FIG. 2A. In FIG. 2B, the numerical value 1.6, 1.9, 2.1and 2.3 indicate concrete examples of time points at which currentvalues I used for calculating the discriminating parameter are measured,the time points indicating the elapsed times after the voltage has beenapplied again.

Next, there will be described a sample discriminating process in theglucose sensor system according to the present invention, namely aconcrete example of a process for discriminating a blood sample and acontrol fluid.

In the concrete example, there were prepared a total of fifteen kinds ofblood samples, whose glucose concentrations were to be measured, bysetting glucose concentration to five different values while settinghematocrit values to three different values and then combining one ofthe glucose concentration to one of the hematocrit values. That is, theglucose concentrations were set to five kinds of 100 mg/dL, 200 mg/dL,300 mg/dL, 400 mg/dL and 500 mg/dL. The hematocrit values were set tothree kinds of 25%, 45% and 65%.

Meanwhile, as control fluids, there were prepared aqueous solutions inwhich PVP (polyvinyl pyrrolidone), which was a hydrophilic polymer, andglucose were dissolved in water. The glucose concentration of theaqueous solutions were set to two kinds of 85 mg/dL and 260 mg/dL. Theviscosity of each of the control fluids was relatively higher.

As to each of the seventeen kinds of samples, current values weremeasured while applying a voltage between the both silver leads 4,5,further between the both electrodes 1,2 in the pattern or process shownin FIG. 2A. The current values were measured at every 0.1 seconds. Thus,four current values at four time points shown in FIG. 2B were selectedfrom the current values which had been selected at every 0.1 seconds, asthe current values required for defining the discrimination function.That is, these are the current value at 1.6 seconds past the startingpoint of the second voltage application (I_(1.6)), the current value at1.9 seconds past the same (I_(1.9)), the current value at 2.1 secondspast the same (I_(2.1)) and the current value at 2.3 seconds past thesame (I_(2.3)). These four time points or current values were obtainedby means of the following procedure.

As described below, the values of I/ΔI were calculated at every 0.1seconds using the resultant data of the current values measured as toall of the seventeen samples described above. Hereupon, the differentialof the current value ΔI was |I_(t)−I_(t+Δt)|. The time difference Δt wasset to 0.5 seconds in order to ensure the repeatability of I/ΔI. Thus,each I/ΔI was classified into two groups, namely the group A as to themeasurement for the blood samples and the group B as to the measurementfor the control fluids. Further, the average value for each of thegroups (Aavg,Bavg) was calculated. Then, the point, in which the twoaverage values were most apart from each other, was selected.

Sample A ₁ =I/ΔI _(0.1) , I/ΔI _(0.2) , I/ΔI _(0.3) , . . . , . . . , .. . , . . . , . . . , I/ΔI _(4.9) , I/ΔI _(5.0)

Sample A ₂ =I/ΔI _(0.1) , I/ΔI _(0.2) , I/ΔI _(0.3) , . . . , . . . , .. . , . . . , . . . , I/ΔI _(4.9) , I/ΔI _(5.0)

Sample A _(x) =I/ΔI _(0.1) , I/ΔI _(0.2) , I/ΔI _(0.3) , . . . , . . . ,. . . , . . . , . . . I/ΔI _(4.9) , I/ΔI _(5.0)

Sample B ₁ =I/ΔI _(0.1) , I/ΔI _(0.2) , I/ΔI _(0.3) , . . . , . . . , .. . , . . . , . . . I/ΔI _(4.9) , I/ΔI _(5.0)

Sample B ₂ =I/ΔI _(0.1) , I/ΔI _(0.2) , I/ΔI _(0.3) , . . . , . . . , .. . , . . . , . . . I/ΔI _(4.9) , I/ΔI _(5.0)

Sample B _(y) =I/ΔI _(0.1) , I/ΔI _(0.2) , I/ΔI _(0.3) , . . . , . . . ,. . . , . . . , . . . , I/ΔI _(4.9) , I/ΔI _(5.0)

Sample Aavg=I/ΔI _(0.1) , I/ΔI _(0.2) , I/ΔI _(0.3) , . . . , . . . , .. . , . . . , . . . , I/ΔI _(4.9) , I/ΔI _(5.0)

Sample Bavg=I/ΔI _(0.1) , I/ΔI _(0.2) , I/ΔI _(0.3) , . . . , . . . , .. . , . . . , . . . , I/ΔI _(4.9) , I/ΔI _(5.0)

In consequence, the point I_(1.6) was selected, in which two groups,namely the group A as to the measurement for the blood samples and thegroup B as to the measurement for the control fluids, were most apartfrom each other. Hereupon, the current value at 1.6 seconds past thestarting point of the second voltage application (I_(1.6)) and thecurrent value at 2.1 seconds past the same (I_(2.1)) are required as thecurrent values for defining the discrimination function I/ΔI_(1.6).

Moreover, in the case that differential of the current value ΔI was|I_(t)−I_(t−Δt)|, also, data processing as same as that of theabove-mentioned case was performed. In this case, the time difference Δtwas set to 0.4 seconds or more in order to ensure the repeatability ofI/ΔI. In consequence, the point I_(2.3) was selected, in which twogroups, namely the group A as to the measurement for the blood samplesand the group B as to the measurement for the control fluids, were mostapart from each other. Hereupon, the current value at 2.3 seconds pastthe starting point of the second voltage application (I_(2.3)) and thecurrent value at 1.9 seconds past the same (I_(1.9)) are required as thecurrent values for defining the discrimination function I/ΔI_(2.3). Bycombining the results with the above-mentioned results, it became clearthat the inclination of the current at nearly 2.0 seconds past thestarting point of the second voltage application was most effective todiscriminate the samples.

Even in the case that only one discriminating parameter in theabove-mentioned two kinds of discriminating parameters is used, thesamples can be discriminated by defining a discrimination functionhaving one independent variable. However, it may remain such a problemthat the accuracy of the discrimination is lowered a little. The reason,why ΔI is calculated in the two way as described above, is nearly asfollows.

That is, in the sense of calculating the inclination of the current atnearly 2.0 seconds past the starting point of the second voltageapplication, the same effect may be obtained in either discriminatingparameter. However, due to the property of the wave shape, there is sucha tendency that the repeatability for the blood sample becomes better inthe former while the repeatability for the control fluid becomes betterin the latter. In consequence, the discriminating effect becomes bestwhen two discriminating parameters are used.

Thus, the discriminating parameter used in the following calculation wascalculated using the measured current values by means of the followingExpression 2 and Expression 3.

I/ΔI _(1.6) =I _(1.6) /|I _(1.6) −I _(2.1)|  Expression 2

I/ΔI _(2.3) =I _(2.3) /|I _(2.3) −I _(1.9)|  Expression 3

Based on the whole measured results as to the seventeen kinds of samplesdescribed above, I/ΔI_(1.6) and I/ΔI_(2.3) were calculated so that thediscrimination function for discriminate the samples was defined. Thedefined discrimination function is indicated by the following Expression4.

Z=8.3014×|I/ΔI _(1.6)|+10.4381×|I/ΔI _(2.3)|−124.6603  Expression 4

Hereinafter, a process for leading the discrimination function will bedescribed with reference to FIG. 3.

FIG. 3 is a graph obtained by plotting a group of discriminatingparameters which are calculated from the measured results as to the twogroups A and B, wherein the position of the horizontal axis denotes|I/ΔI_(1.6)| while the position of the vertical axis denotes|I/ΔI_(2.3)|. In this case, a first-order function, which can bestseparate the two groups of the discriminating parameters, is indicatedby the following linear equation.

Z=a ₁ ×x ₁ ×a ₂ +x ₂ +a ₀

Hereupon, a straight line 13, which indicates the boundary between thetwo groups of the discriminating parameters, is a graph in the case ofZ=0, namely the following expression.

0=a ₁ ×x ₁ +a ₂ ×x ₂ +a ₀

Therefore, dividing the groups of discriminating parameters into twogroups by the straight line 13 means dividing the groups ofdiscriminating parameters into two groups in accordance with whether thevalue of Z according to the above-mentioned first-order function is plusor minus. In the case that the groups of discriminating parameters aredivided into two groups by a curved line instead of the straight line,there may be used a high-order function for the discriminatingparameters or independent variables, for example nth-order function(n=2, 3, 4 . . . ), as the discrimination function. Meanwhile, in thecase that the number of the discriminating parameters is three, thediscrimination function is indicated by the following expression.

Z=a ₁ ×x ₁ +a ₂ ×x ₂ +a ₃ ×x ₃ +a ₀

In this case, the boundary is indicated by the graph of Z=0, namely thefollowing expression.

0=a ₁ ×x ₁ +a ₂ ×x ₂ +a ₃ ×x ₃ +a ₀

This expression indicates a plane surface in a three dimensional space.In general, if the number of the discriminating parameters is P, theboundary, which is a (p−1) dimensional surface in a P dimensional space,is indicated by the following expression.

0=a ₁ ×x ₁ +a ₂ ×x ₂ +a ₃ ×x ₃ + . . . a _(p) ×x _(p) a ₀

The samples were discriminated on the basis of the discriminating indexvalue Z, which was calculated by substituting the value of thediscriminating parameter obtained by the measurement into thediscrimination function indicated by Expression 4 described above. Inthis case, based on the discriminating index value Z, the samples werediscriminated in accordance with the following rule.

(1) In the case of Z<−8, it is judged that the sample is a controlfluid.

(2) In the case of −8≦Z≦8, the judgement is not performed while it isdecided that the sample is un-decidable.

(3) In the case of Z>8, it is judged that the sample is blood.

In the case of using the discrimination function indicated by Expression4, which is defined in the concrete example, the rate of the erroneousjudgement is 0.011%. Hereupon, the rate of the erroneous means aprobability of the erroneous judgement when the kinds of the samples arediscriminated in accordance with the sign while assuming that thediscriminating index values Z of the two groups to be discriminated aredistributed with the normal distribution. That is, in this concreteexample, it is the average value of the probability of becoming Z<0 whenthe control fluids are measured and the probability of becoming Z≧0 whenthe blood samples are measured.

Table 1 described below shows results when the judgement is performedonly on the basis of the sign of the discriminating index value withoutsetting the un-decidable area, namely results in the case that it isjudged to be a control fluid if Z≧0 while it is judged to be a bloodsample if Z<0.

TABLE 1 Case without un-decidable area Results based on index value ZSample Control fluids Blood samples Control fluids 832 2 Blood samples 13348

On the other hand, when the un-decidable area is provided, no erroneousjudgements occur at all. Table 2 described below shows results when thesamples are not discriminated in the case of −8≦Z≦8 as described above.In this case, the probability of being judged to be not-decidable is1.3% as to the control fluids, while it is 0.1% as to the blood samples.Hereupon, if it is judged to be un-decidable, into the system, the usermust input such information that which sample the user has measured bythe manual operation.

TABLE 2 Case with un-decidable area Results based on index value ZSample Control fluids Un-decidable Blood samples Control fluids 823 11 0Blood samples 0 5 344

If kinds of samples are different from the above-mentioned ones, thecurrent wave shapes are also different. Therefore, it is necessary tochange the time points or numbers of calculating the discriminatingparameters (I/ΔI) of the samples in accordance with the kinds of thesamples.

As described above, in the biosensor system constructed according to thepresent invention, kinds of samples to be measured can be automaticallyrecognized by the sensor system without charging any expenses to users,and then the results may be informed to the users. Further, in thebiosensor system constructed as the above, when control fluids aremeasured, the users can recognize the states of the sensor systemwithout troublesome works.

Industrial Applicability

As described above, the sample discriminating method according to thepresent invention is useful as a discriminating method in a sensorsystem for measuring the concentration of a target substance containedin a sample, and particularly is suitable for using in a biosensor forquantitating the concentration of glucose, cholesterol or the likecontained in a body fluid such as blood by measuring the electriccurrent.

What is claimed is:
 1. A method of discriminating a sample for a sensorsystem which quantitates a concentration of a target substance containedin the sample by measuring electric current, said method comprising thesteps of: using a ratio of a measured current value to a timedifferential value of the current value as a discriminating parameter;defining a discrimination function for discriminating kinds of aplurality of samples, said discrimination function using saiddiscriminating parameter as an independent variable; using a numericvalue obtained by substituting the value of said discriminatingparameter into said discrimination function, as a discriminating index;and discriminating the kind of any one of the plurality of samples basedon said discriminating index, wherein said discrimination function isdefined by means of an expression of a high degree for saiddiscriminating parameter.
 2. The method according to claim 1, whereinsaid discrimination function is defined by means of an expression usinga plurality of said discriminating parameters.
 3. The method accordingto claim 1, wherein the kinds of the samples to be discriminated are abody fluid and a control fluid.
 4. The method according to claim 3,comprising the steps of: judging whether said sensor system is operatingproperly or not based on a quantitated value of the concentration of thetarget substance; and indicating a result of the judging step.
 5. Themethod according to claim 1, further comprising the step of: indicatingthat the step of discriminating the kind of the sample has not beenautomatically performed when the discriminating index is within apredetermined range such that it is difficult to discriminate the kindof the sample.
 6. The method according to claim 5, further comprisingthe step of: designating that a manual operation is required when thediscriminating index is within the predetermined range such that it isdifficult to discriminate the kind of the sample.